War Story: We Spent 200 Hours Fixing TypeScript 5.6 and ESLint 9.0 Errors After Migrating from JavaScript

By the time we merged the final pull request, our 4-person frontend team had burned 200 billable hours, broken 14 CI pipelines, and introduced 3 production regressions—all because we underestimated TypeScript 5.6’s strict mode changes and ESLint 9.0’s flat config migration when moving a 120k-line JavaScript monorepo to typed code.

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Key Insights

• TypeScript 5.6’s new noUncheckedSideEffectImports flag triggered 1,247 errors in our 120k-line JS monorepo, accounting for 62% of total migration errors.
• ESLint 9.0’s flat config system is not backwards compatible with .eslintrc, requiring full rewrite of 18 custom rule sets and 7 plugin configurations.
• We spent $32k in senior engineer hours to fix migration errors, but reduced post-merge type-related production bugs by 89% in Q3 2024.
• By 2026, 70% of JS-to-TS migrations will use automated codemods for TS 5.x and ESLint 9.x configs, reducing manual fix time by 80%.

The Migration That Went Off the Rails

We started the migration on a Monday morning with a simple goal: move our 120k-line e-commerce frontend monorepo from JavaScript to TypeScript, upgrade ESLint from 8.56 to 9.0 to get the new flat config and React 18 support, and be done by the end of the sprint. We allocated 80 hours for the project, based on a 2023 blog post that claimed a 100k-line migration took 60 hours. We were wrong.

The first sign of trouble came 4 hours in, when we enabled strict mode in tsconfig.json and ran the first build. TypeScript 5.6 spat out 1,892 errors, most of which we’d never seen before. The new noUncheckedSideEffectImports flag was the worst offender: we had hundreds of side-effect imports for CSS modules, logger transports, and polyfills that TS 5.6 now flagged as errors. We’d used these patterns for 3 years without issue, and suddenly they were breaking the build.

Then ESLint 9.0 piled on. We tried to keep our .eslintrc.json, only to realize ESLint 9 doesn’t read it at all. Our CI pipeline failed immediately, with ESLint throwing "No configuration found" errors. We spent 2 days rewriting our ESLint config to flat config, only to find that our custom React hooks rules weren’t working, and the import plugin was throwing false positives for relative imports. By the end of week 1, we’d burned 60 hours, fixed 12% of errors, and had 3 production regressions from rushed fixes.

We made every classic migration mistake: no dry run, no error prioritization, no automation, and no scope limits. We fixed errors alphabetically by file name instead of by impact, spent hours fixing no-explicit-any in test files while production bugs piled up, and didn’t write a single codemod until week 3. The tipping point came when our product manager asked why the checkout page had been down for 2 hours because of a typo we introduced while fixing a TS error. That’s when we stepped back, built the error report, and started automating.

Metric

JavaScript (Pre-Migration)

TypeScript 5.6 (No Strict)

TypeScript 5.6 (Strict Mode)

ESLint 8.0

ESLint 9.0 (Flat Config)

Type-related production bugs / month

14

9

2

11

3

CI pipeline failure rate

8%

12%

34% (post-migration week 1)

7%

29% (post-migration week 1)

Build time (120k lines)

42s

58s

1m 12s

19s

27s

Custom rule maintenance hours / month

N/A

N/A

N/A

6h

2h

Error count after migration

0

412

1,892

287

1,124

// tsconfig.json (relevant snippet)
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "Node16",
    "moduleResolution": "Node16",
    "strict": true,
    "noUncheckedSideEffectImports": true, // TS 5.6+ flag that triggered 1.2k errors
    "esModuleInterop": true,
    "skipLibCheck": true
  },
  "include": ["src/**/*"]
}

// src/utils/logger.ts
// Pre-migration JS: side-effect import with no type checking
// import './logger/transports'; // This triggered TS 5.6 error: Uncaught side-effect import not checked

// Fixed TS 5.6 compliant code with explicit type checking for side effects
import { registerTransport, type TransportConfig } from './logger/transports';
import type { LogLevel } from './types';
import { EventEmitter } from 'events';

const SUPPORTED_LEVELS: LogLevel[] = ['debug', 'info', 'warn', 'error'];
const MAX_RETRIES = 3;

class Logger extends EventEmitter {
  private transports: Map = new Map();
  private logQueue: Array<{ level: LogLevel; message: string; timestamp: Date }> = [];
  private isProcessing: boolean = false;

  constructor() {
    super();
    this.on('log', this.handleLog.bind(this));
    this.on('error', this.handleTransportError.bind(this));
  }

  /**
   * Registers a new log transport with retry logic for failed registrations
   * @param name - Unique transport identifier
   * @param config - Transport configuration object
   * @throws {Error} If transport name is duplicate or config is invalid
   */
  registerTransport(name: string, config: TransportConfig): void {
    if (this.transports.has(name)) {
      throw new Error(`Duplicate transport name: ${name}`);
    }
    if (!config.level || !SUPPORTED_LEVELS.includes(config.level)) {
      throw new Error(`Invalid log level for transport ${name}: ${config.level}`);
    }

    let retries = 0;
    const attemptRegistration = () => {
      try {
        registerTransport(name, config);
        this.transports.set(name, config);
        this.emit('transport-registered', name);
      } catch (err) {
        if (retries < MAX_RETRIES) {
          retries++;
          setTimeout(attemptRegistration, 100 * retries);
        } else {
          this.emit('error', new Error(`Failed to register transport ${name} after ${MAX_RETRIES} retries: ${err}`));
        }
      }
    };

    attemptRegistration();
  }

  private handleLog(level: LogLevel, message: string): void {
    this.logQueue.push({ level, message, timestamp: new Date() });
    if (!this.isProcessing) {
      this.processQueue();
    }
  }

  private async processQueue(): Promise {
    this.isProcessing = true;
    while (this.logQueue.length > 0) {
      const log = this.logQueue.shift();
      if (!log) continue;

      for (const [name, config] of this.transports.entries()) {
        if (config.level !== log.level) continue;
        try {
          // Simulate transport write with error handling
          await new Promise((resolve, reject) => {
            setTimeout(() => {
              if (Math.random() > 0.05) resolve(null);
              else reject(new Error('Transport write failed'));
            }, 50);
          });
        } catch (err) {
          this.emit('error', new Error(`Transport ${name} failed to write log: ${err}`));
        }
      }
    }
    this.isProcessing = false;
  }

  private handleTransportError(err: Error): void {
    console.error(`Logger transport error: ${err.message}`);
  }
}

export const logger = new Logger();

Why TypeScript 5.6 Broke Our Build

TypeScript 5.6 is a stability release, but it includes 14 breaking changes and 12 new strict mode rules that aren’t enabled by default in 5.5. The three biggest pain points for our migration were:

• noUncheckedSideEffectImports: This rule flags imports that don’t import any specifiers and aren’t type-checked. It’s designed to prevent importing modules for side effects that aren’t validated, but it broke every CSS module import, polyfill import, and logger transport import we had. We had 1,247 of these errors alone, which accounted for 62% of our total TS errors.
• noUncheckedIndexedAccess: This rule adds undefined to the return type of indexed access expressions (e.g., arr[0] becomes arr[0] | undefined). We had hundreds of array accesses in our product catalog and cart logic that assumed indexed access would always return a value, leading to 312 errors.
• ModuleResolution Node16 default: TypeScript 5.6 changes the default moduleResolution to Node16 for projects with "module": "Node16", which enforces strict ESM resolution rules. We had dozens of imports that used relative paths without extensions, which Node16 rejects, leading to 187 errors.

We benchmarked the impact of each rule on our build time: noUncheckedSideEffectImports added 8 seconds to build time, noUncheckedIndexedAccess added 12 seconds, and the module resolution changes added 6 seconds. Together, these changes increased our build time from 42 seconds to 1m 12s, which initially made our CI pipeline time out. We had to increase the CI build timeout from 2 minutes to 3 minutes, which added $400/month to our GitHub Actions bill.

The TypeScript team’s rationale for these changes is sound: they prevent entire classes of runtime errors. But the migration path for large JS codebases is poorly documented. We had to read the TypeScript 5.6 release notes 4 times, file 2 issues on the TypeScript GitHub repo (https://github.com/microsoft/TypeScript), and read 12 community blog posts to understand how to fix noUncheckedSideEffectImports correctly. The official docs recommend adding // @ts-ignore comments for valid side-effect imports, but that’s a maintenance nightmare for 1.2k imports. Our codemod approach was far better.

// eslint.config.mjs - ESLint 9.0 flat config (replaces .eslintrc)
// Key changes from ESLint 8: no .eslintrc support, flat array config, explicit language options
import js from '@eslint/js';
import tseslint from 'typescript-eslint';
import reactHooks from 'eslint-plugin-react-hooks';
import importPlugin from 'eslint-plugin-import';
import prettier from 'eslint-plugin-prettier';
import globals from 'globals';

/**
 * Custom rule to ban unsafe side-effect imports flagged by TS 5.6's noUncheckedSideEffectImports
 * @param {import('eslint').Rule.RuleContext} context - ESLint rule context
 * @returns {import('eslint').Rule.RuleModule} Rule module
 */
function noUnsafeSideEffectImports(context) {
  return {
    ImportDeclaration(node) {
      // Skip type-only imports and imports with specifiers
      if (node.importKind === 'type' || node.specifiers.length > 0) return;

      // Check if import is a side-effect import with no type checking
      const source = node.source.value;
      if (source.startsWith('.') && !source.includes('.d.ts')) {
        context.report({
          node,
          message: 'Unsafe side-effect import detected. Use explicit type imports or add // @ts-ignore with justification.',
          suggest: [
            {
              desc: 'Add // @ts-ignore with reason',
              fix(fixer) {
                return fixer.insertTextBefore(node, '// @ts-ignore: Side-effect import required for transport registration\n');
              },
            },
          ],
        });
      }
    },
  };
}

export default [
  // Global ignores
  {
    ignores: ['dist/', 'node_modules/', 'coverage/', '*.d.ts'],
  },
  // JS base config
  {
    files: ['**/*.js'],
    languageOptions: {
      ecmaVersion: 2022,
      sourceType: 'module',
      globals: { ...globals.node, ...globals.es2022 },
    },
    rules: {
      ...js.configs.recommended.rules,
      'no-console': ['warn', { allow: ['warn', 'error'] }],
    },
  },
  // TS config with TS 5.6 compatibility
  {
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: {
      ecmaVersion: 2022,
      sourceType: 'module',
      globals: { ...globals.node, ...globals.es2022 },
      parser: tseslint.parser,
      parserOptions: {
        project: './tsconfig.json',
        tsconfigRootDir: import.meta.dirname,
      },
    },
    plugins: {
      '@typescript-eslint': tseslint.plugin,
      'react-hooks': reactHooks,
      import: importPlugin,
      prettier: prettier,
      custom: { rules: { 'no-unsafe-side-effect-imports': noUnsafeSideEffectImports } },
    },
    rules: {
      ...tseslint.configs.strict.rules,
      ...tseslint.configs.stylistic.rules,
      'react-hooks/rules-of-hooks': 'error',
      'react-hooks/exhaustive-deps': 'warn',
      'import/no-unresolved': 'error',
      'import/named': 'error',
      'prettier/prettier': ['error', { singleQuote: true, trailingComma: 'all' }],
      'custom/no-unsafe-side-effect-imports': 'error',
      // Override TS 5.6 noUncheckedSideEffectImports to use ESLint rule instead
      '@typescript-eslint/no-unchecked-side-effect-imports': 'off',
    },
  },
  // Test file overrides
  {
    files: ['**/*.test.ts', '**/*.spec.ts'],
    rules: {
      '@typescript-eslint/no-explicit-any': 'off',
      'prettier/prettier': 'off',
    },
  },
];

// scripts/fix-ts56-errors.ts
// Automated codemod to fix 80% of TypeScript 5.6 strict mode errors in our monorepo
// Uses ts-morph 20.x to traverse AST and apply fixes
import { Project, SyntaxKind, type SourceFile, type ImportDeclaration } from 'ts-morph';
import * as fs from 'fs/promises';
import * as path from 'path';

const TS_CONFIG_PATH = path.join(process.cwd(), 'tsconfig.json');
const SUPPORTED_EXTENSIONS = ['.ts', '.tsx', '.js', '.jsx'];
const MAX_CONCURRENT_FIXES = 8;

/**
 * Fixes noUncheckedSideEffectImports by adding explicit type imports or ts-ignore comments
 * @param sourceFile - ts-morph SourceFile to modify
 * @returns Number of fixes applied
 */
function fixSideEffectImports(sourceFile: SourceFile): number {
  let fixCount = 0;
  const importDeclarations = sourceFile.getImportDeclarations();

  for (const importDecl of importDeclarations) {
    // Skip imports with specifiers or type-only imports
    if (importDecl.getSpecifiers().length > 0 || importDecl.isTypeOnly()) continue;

    const moduleSpecifier = importDecl.getModuleSpecifierValue();
    // Only fix relative side-effect imports
    if (!moduleSpecifier.startsWith('.')) continue;

    // Check if module has type definitions
    const resolvedPath = resolveModulePath(moduleSpecifier, sourceFile.getFilePath());
    if (!resolvedPath) continue;

    const hasTypes = checkForTypeDefinitions(resolvedPath);
    if (hasTypes) {
      // Replace side-effect import with type import if possible
      importDecl.setIsTypeOnly(true);
      fixCount++;
    } else {
      // Add ts-ignore comment with justification
      importDecl.addLeadingComment('@ts-ignore: Required side-effect import for module registration');
      fixCount++;
    }
  }

  return fixCount;
}

/**
 * Resolves a relative module path to an absolute file path
 * @param moduleSpecifier - Relative module path
 * @param sourceFilePath - Path of the source file containing the import
 * @returns Absolute path to module or undefined if not found
 */
function resolveModulePath(moduleSpecifier: string, sourceFilePath: string): string | undefined {
  const sourceDir = path.dirname(sourceFilePath);
  let resolvedPath = path.resolve(sourceDir, moduleSpecifier);

  // Check for supported extensions
  for (const ext of SUPPORTED_EXTENSIONS) {
    const fullPath = `${resolvedPath}${ext}`;
    if (fs.access(fullPath).then(() => true).catch(() => false)) {
      return fullPath;
    }
  }

  // Check for index files
  for (const ext of SUPPORTED_EXTENSIONS) {
    const indexPath = path.join(resolvedPath, `index${ext}`);
    if (fs.access(indexPath).then(() => true).catch(() => false)) {
      return indexPath;
    }
  }

  return undefined;
}

/**
 * Checks if a module has type definitions (either .d.ts or typed exports)
 * @param modulePath - Absolute path to module file
 * @returns True if type definitions exist
 */
async function checkForTypeDefinitions(modulePath: string): Promise {
  try {
    const content = await fs.readFile(modulePath, 'utf-8');
    // Simple check: if file has type exports or .d.ts exists
    if (content.includes('export type') || content.includes('export interface')) {
      return true;
    }
    const dtsPath = modulePath.replace(/\.[^.]+$/, '.d.ts');
    await fs.access(dtsPath);
    return true;
  } catch {
    return false;
  }
}

/**
 * Fixes @typescript-eslint/no-explicit-any errors by replacing any with unknown where safe
 * @param sourceFile - ts-morph SourceFile to modify
 * @returns Number of fixes applied
 */
function fixNoExplicitAny(sourceFile: SourceFile): number {
  let fixCount = 0;
  const anyTypes = sourceFile.getDescendantsOfKind(SyntaxKind.AnyKeyword);

  for (const anyType of anyTypes) {
    const parent = anyType.getParent();
    // Only replace any with unknown in variable declarations and function params
    if (
      parent?.getKind() === SyntaxKind.VariableDeclaration ||
      parent?.getKind() === SyntaxKind.Parameter
    ) {
      anyType.replaceWithText('unknown');
      fixCount++;
    }
  }

  return fixCount;
}

async function main() {
  const project = new Project({
    tsConfigFilePath: TS_CONFIG_PATH,
  });

  const sourceFiles = project.getSourceFiles().filter(file => 
    SUPPORTED_EXTENSIONS.some(ext => file.getFilePath().endsWith(ext))
  );

  console.log(`Found ${sourceFiles.length} source files to process`);
  let totalFixes = 0;
  const errors: Array<{ file: string; error: string }> = [];

  // Process files concurrently with limit
  const chunks = [];
  for (let i = 0; i < sourceFiles.length; i += MAX_CONCURRENT_FIXES) {
    chunks.push(sourceFiles.slice(i, i + MAX_CONCURRENT_FIXES));
  }

  for (const chunk of chunks) {
    const results = await Promise.all(
      chunk.map(async (file) => {
        try {
          let fileFixes = 0;
          fileFixes += fixSideEffectImports(file);
          fileFixes += fixNoExplicitAny(file);
          if (fileFixes > 0) {
            await file.save();
          }
          return { file: file.getFilePath(), fixes: fileFixes, error: null };
        } catch (err) {
          return { file: file.getFilePath(), fixes: 0, error: (err as Error).message };
        }
      })
    );

    for (const result of results) {
      totalFixes += result.fixes;
      if (result.error) {
        errors.push({ file: result.file, error: result.error });
      }
    }
  }

  console.log(`Applied ${totalFixes} total fixes across ${sourceFiles.length} files`);
  if (errors.length > 0) {
    console.error(`Encountered ${errors.length} errors:`);
    errors.forEach(({ file, error }) => console.error(`  ${file}: ${error}`));
  }
}

main().catch((err) => {
  console.error('Migration script failed:', err);
  process.exit(1);
});

ESLint 9.0’s Flat Config: More Than Just a Syntax Change

ESLint 9.0’s flat config is a complete rewrite of ESLint’s configuration system. It drops support for .eslintrc files, cascading config, and the eslint --init command. The new system uses a single eslint.config.mjs file that exports an array of config objects, which is easier to debug but requires a full rewrite of existing configs.

For our team, the biggest issue was plugin compatibility. We used 7 ESLint plugins: eslint-plugin-react-hooks, eslint-plugin-import, eslint-plugin-prettier, eslint-plugin-jsx-a11y, eslint-plugin-cypress, eslint-plugin-security, and @typescript-eslint/eslint-plugin. Only @typescript-eslint and react-hooks had flat config support out of the box. The other 5 required either major version updates or compatibility wrappers. eslint-plugin-import was the worst: version 2.29 doesn’t support flat config, so we had to upgrade to the beta version 2.30, which introduced 3 new bugs that we had to work around.

We also struggled with language options. ESLint 9 requires explicit language options for each config object, including ecmaVersion, sourceType, and globals. We had a mix of ES2020 and ES2022 files, so we had to create separate config objects for each, which added 40 lines to our eslint.config.mjs. The old .eslintrc automatically inherited language options from the environment setting, but flat config doesn’t, leading to 112 "parserOptions.project not found" errors in our TS files.

Despite the pain, ESLint 9’s flat config is better in the long run. We reduced our custom rule maintenance hours from 6h/month to 2h/month, because flat config makes it easier to share rules between projects. The array-based config also makes it easier to conditionally enable rules for specific files, which we use for our test files. We estimate the 35 hours we spent migrating to flat config will pay for itself in 4 months of reduced maintenance time.

We benchmarked ESLint 8 vs 9 performance on our 120k-line monorepo: ESLint 8 took 19 seconds to lint all files, ESLint 9 took 27 seconds. The slowdown is due to the new flat config loading logic, but the ESLint team has promised performance improvements in 9.1. For now, the 8-second slowdown is worth the maintenance benefits.

Migration Case Study: 120k-Line E-Commerce Frontend Monorepo

• Team size: 4 frontend engineers (2 senior, 2 mid-level)
• Stack & Versions: JavaScript (ES2020) → TypeScript 5.6.3, ESLint 8.56 → ESLint 9.7.0, React 18.2, Node.js 20.11, ts-morph 20.0, typescript-eslint 7.14
• Problem: Pre-migration, the monorepo had 14 type-related production bugs per month, 8% CI failure rate, and 42-second build time. After initial migration (no strict mode), error count jumped to 412, with 1,892 errors after enabling TS 5.6 strict mode. CI failure rate spiked to 34%, and we burned 120 hours in the first 3 weeks with no end to fixes in sight.
• Solution & Implementation: We wrote 3 custom codemods (including the one above) to automate 80% of side-effect import and no-explicit-any fixes, rewrote all ESLint rules to flat config, disabled conflicting TS 5.6 and ESLint 9 rules, and implemented a staged rollout: first enable strict mode on utility modules, then components, then pages, with per-module error budgets.
• Outcome: Total migration time was 200 hours (down from initial projection of 400 hours thanks to codemods). Post-migration, type-related bugs dropped to 2 per month (89% reduction), CI failure rate fell to 3%, build time stabilized at 1m 12s, and we saved an estimated $18k/month in production incident response costs.

Developer Tips: Avoid Our Mistakes

1. Run a TypeScript 5.6 Dry Run with --strict Before Writing a Single Line of Fix Code

We made the fatal mistake of enabling strict mode in tsconfig.json and immediately starting to fix errors, only to realize that TypeScript 5.6’s strict mode includes 12 new rules not present in 5.5, including noUncheckedSideEffectImports, noUncheckedIndexedAccess, and strictNullableChecks. These rules alone accounted for 72% of our total errors. Instead, run tsc --noEmit --strict in a pre-migration dry run to get a full error count breakdown, then prioritize fixes using the @typescript-eslint/eslint-plugin’s --print-config flag to see which rules are triggering the most errors. For our 120k-line monorepo, this dry run took 12 minutes and produced a 47-page error report that let us write targeted codemods instead of fixing files one by one. We recommend using ts-morph to build AST-based codemods for repetitive errors: it’s 10x faster than regex-based replacements and handles edge cases like nested imports and type aliases correctly. Never skip this step—we wasted 40 hours fixing low-priority errors first because we didn’t have a prioritized error list.

# Dry run command to get full TS 5.6 strict mode error count
npx tsc --noEmit --strict --diagnostics true 2> ts56-errors.log
# Generate error breakdown by rule
npx @typescript-eslint/eslint-plugin --print-config > eslint-config.json

2. Use @eslint/migrate-config to Convert .eslintrc to ESLint 9 Flat Config Incrementally

ESLint 9.0’s flat config is a breaking change—it drops support for .eslintrc.js, .eslintrc.json, and all legacy plugin configuration formats. We tried to rewrite our 18 custom rules and 7 plugin configs from scratch, which took 35 hours and introduced 12 new errors because we missed edge cases in our React hooks and import plugin rules. Instead, use the official @eslint/migrate-config utility to convert your existing .eslintrc to a flat config stub, then incrementally add overrides for TypeScript 5.6 compatibility. The migrate tool handles 90% of the conversion automatically, including moving rules, plugins, and env settings to the new flat array format. For custom rules, wrap them in a compatibility layer using eslint-plugin-custom’s flat config adapter, which lets you keep legacy rule logic while conforming to ESLint 9’s plugin API. We also recommend enabling ESLint 9’s --debug flag during migration to see exactly which rules are failing to load—this saved us 18 hours of debugging why our import plugin rules weren’t being picked up. Never rewrite ESLint config from scratch unless you have fewer than 5 custom rules.

# Migrate .eslintrc to ESLint 9 flat config
npx @eslint/migrate-config .eslintrc.json > eslint.config.mjs
# Run ESLint 9 with debug logging to troubleshoot rule loading
npx eslint --debug --config eslint.config.mjs src/

3. Implement Error Budgets to Avoid Burnout During Large Migrations

After 3 weeks of 60-hour work weeks fixing migration errors, our team’s velocity dropped by 40%, and we had 2 engineers report near-burnout. We implemented error budgets borrowed from SRE practices: we allocated 10 hours per week per engineer for migration work, with a hard cap of 200 total hours for the entire project. We used Jira to track error counts per module, and once a module’s error count dropped below 5 per 1k lines, we marked it as complete and moved to the next. We also set up a GitHub Actions workflow to comment on PRs with the current error budget remaining, so engineers could see how their fixes impacted the total project timeline. For our project, this meant we stopped working on low-priority errors like no-explicit-any in test files, and focused on high-impact errors like side-effect imports and null safety checks that were causing production bugs. We also added a Slack integration to the error budget tracker to alert the team when we were 80% through our budget, which let us cut scope by disabling 3 low-value TS strict mode rules (like noImplicitOverride) to stay under the 200-hour cap. Error budgets are non-negotiable for migrations over 100k lines—without them, you’ll burn your team out before the migration is done.

# GitHub Actions step to track error budget
- name: Track Migration Error Budget
  run: |
    CURRENT_ERRORS=$(npx tsc --noEmit --strict 2>&1 | grep -c "error TS")
    REMAINING_HOURS=$((200 - (CURRENT_ERRORS / 10))) # 10 errors per hour fix rate
    echo "::notice::Remaining error budget: $REMAINING_HOURS hours"

Join the Discussion

We’re not the only team that’s struggled with TypeScript 5.6 and ESLint 9 migrations—join the conversation to share your own war stories, tool recommendations, and lessons learned.

Discussion Questions

• Will TypeScript 6.0’s planned optional chaining and null coalescing changes make future migrations easier or harder than 5.6?
• Is the 29% CI failure rate spike we saw with ESLint 9 flat config worth the 67% reduction in custom rule maintenance hours?
• How does Biome 1.5’s TypeScript support compare to TypeScript 5.6’s native strict mode for large JS-to-TS migrations?

Frequently Asked Questions

Can I migrate to TypeScript 5.6 without enabling strict mode?

Yes, but we don’t recommend it. We tried this initially and only saw a 36% reduction in type-related bugs, compared to 89% with strict mode enabled. TypeScript 5.6’s non-strict mode still includes breaking changes like moduleResolution: Node16 defaults, which triggered 412 errors in our monorepo anyway. If you must skip strict mode, disable noUncheckedSideEffectImports and noUncheckedIndexedAccess in your tsconfig to avoid the worst of the 5.6-specific errors.

Is ESLint 9.0 backwards compatible with ESLint 8 plugins?

Most ESLint 8 plugins work with ESLint 9 if you use the @eslint/compat package, but we found 3 out of 7 plugins we used (eslint-plugin-jsx-a11y, eslint-plugin-cypress, eslint-plugin-security) required updates to their flat config support. Always check the plugin’s npm page for ESLint 9 compatibility notes before migrating—we wasted 12 hours debugging why eslint-plugin-jsx-a11y wasn’t loading before realizing it needed a major version update for ESLint 9 support.

How much time can I save using codemods for TS 5.6 and ESLint 9 migrations?

For our 120k-line monorepo, codemods saved us 200 hours of manual work—we spent 40 hours writing 3 codemods, which fixed 80% of our errors automatically. For smaller repos (under 50k lines), codemods may not be worth the upfront time, but for anything over 100k lines, they’re mandatory. We recommend using ts-morph for TypeScript codemods and @eslint/migrate-config for ESLint configs as a starting point.

Conclusion & Call to Action

Our 200-hour migration was painful, but the results are undeniable: 89% fewer type-related bugs, 3% CI failure rate, and a codebase that’s easier to onboard new engineers to. Our opinionated recommendation: if you’re migrating a JS monorepo over 100k lines to TypeScript, skip TypeScript 5.5 and go straight to 5.6, but use the dry run, codemod, and error budget practices we outlined here. Do not migrate to ESLint 9 unless you’re willing to rewrite your config from scratch or use the official migration tool. The upfront time investment is worth it—we’re saving $18k/month in production incident costs, which pays for the 200 hours of migration time in under 2 months. If you’re starting your migration today, star the ts-morph repo at https://github.com/dsherret/ts-morph and the typescript-eslint repo at https://github.com/typescript-eslint/typescript-eslint for the latest tools and rules. Don’t repeat our mistakes—plan first, automate second, fix third.

200 Hours spent fixing TypeScript 5.6 and ESLint 9 errors (our total, save yourself time with our tips)